Color-magnitude diagram for the clusters in our sample E(B − V) < 1, color coded with their mean iron abundance. The plot on the left displays 103 GCs that contain at least one RRL. The right plot shows the same but for 24 clusters without RRLs. The red lines indicate the position of the theoretical blue and red edge of the instability strip for RRab, with Z = 0.0003, M = 0.7M_⊙ and Y = 0.290, see Sect. 5.